A need presently exists for an effective and efficient means for reducing nitrate and nitrite contaminant levels in groundwater and in other water supplies. A need particularly exists for such a water treatment process which is substantially odor free and which provides a treated water product containing little or no trihalomethanes and containing little or no volatile organic carbon compounds.
Some water treatment methods used heretofore for nitrate removal have involved ion exchange, reverse osmosis, or electrodialysis. Each of these methods, however, simply removes nitrate ions from one water stream and concentrates said ions in a second water stream. The second water stream must then be disposed of.
U.S. Pat. No. 5,211,847 discloses a multistage treatment system for the biological denitrification of secondary effluents from wastewater treatment plants. The system uses an upflow biological reactor containing denitrifying bacterial growths supported on rigid, corrugated plastic panels. Upstream of the biological reactor, an organic carbon nutrient source (e.g., methanol) is added to the water. To obtain the oxygen necessary for consuming the carbon source, the denitrifying bacteria break down nitrate and nitrite contaminants contained in the water stream. The break down of the nitrate and nitrite contaminants yields a nitrogen gas product which is released to the atmosphere.
Before breaking down the nitrate and nitrite contaminants, the bacteria used in the U.S. Pat. No. 5,211,847 system must first consume any free oxygen (i.e., O.sub.2) dissolved in the water stream. The need to biologically consume the dissolved free oxygen increases the amount of carbon source which must be fed to the bacteria and also increases the required size of the biological reactor. Feeding additional organic carbon material to the bioreactor increases the amount of odor produced by the biological process and generally increases the amounts of volatile organic carbons and trihalomethanes contained in the treated water product. Further, the use of additional organic carbon source increases the amount of chlorine which must be used to disinfect the treated water product. Moreover, removing the dissolved free oxygen solely by biological means results in (a) excessive biomass growth, (b) blockage of reactor flow passages, and (c) an increase in the amount of start up time required to achieve full reactor denitrification activity.
U.S. Pat. No. 4,970,000 discloses a biological denitrification system wherein the free oxygen content of the source water stream is reduced prior to delivery to the denitrification reactor. Free oxygen removal is accomplished by either (a) reacting the free oxygen with hydrogen, (b) vacuum degassing, or (c) purging with nitrogen gas.
Each of the oxygen removal techniques disclosed in U.S. Pat. No. 4,970,000 has significant shortcomings. Vacuum degassing requires the use of a vacuum chamber and a vacuum pump. Nitrogen purging increases the nitrogen content of the water stream and thereby increases both the amount of carbon nutrient required and the size of the biological reactor. Hydrogen treatment requires the installation of a hydrogenation vessel. Hydrogen treatment also requires the use of a costly precious metal catalyst which poses potential contamination problems.
The biological denitrification reactor used in the U.S. Pat. 4,970,000 system contains a fluidized sand bed populated with denitrification bacteria. Caking of the reactor sand bed can occur as a result of system shutdown and/or excessive biomass growth. To prevent such caking, it is necessary to intermittently blow air through the sand bed and to stir the sand bed using an agitator. Unfortunately, when air is injected into the reactor bed, the resulting introduction of free oxygen substantially reduces the nitrogen removal efficiency of the reactor. Full nitrogen removal efficiency is not recovered until all of the free oxygen is either consumed or flushed from the reactor.
As with U.S. Pat. 4,970,000, U.S. Pat. No. 4,756,831 discloses a biological denitrification system wherein, prior to introduction into the biological reactor, free oxygen is removed from the water source by oxidation with hydrogen.
Sulfites have been used heretofore in boiler units and in other hot water applications to reduce oxygen levels and thereby alleviate equipment corrosion problems. However, since sulfites react with free oxygen at a much slower rate in cold water than in hot water, sulfites have not been considered heretofore for use as oxygen removal agents in biological denitrification processes.